Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />Services Program Manager with the Parks and Open Space Division. The new pavement <br />widening and extension runoff will release into this ditch matching the pre-developed <br />discharge rates. This ditch continues on to the north where it is released into Hendrick's <br />Park. In other words, the development will be re-directing much of the new pavement <br />improvements and existing upland areas previously draining toward the residents to the <br />west, and instead directing that runoff into Hendrick's park, i.e. these improvements will <br />reduce the runoff draining toward the residents to the west. <br />3.1.2 Private Roadway Runoff <br />Runoff created from the impervious pavement of the private roadway will be treated by <br />mechanical treatment devices in 1 of the 3 separate outfall locations. A majority of the <br />runoff will be detained in underground storage tanks in order to meet the existing <br />discharge rates offsite, thereby meeting flow control requirements. The runoff <br />discharged to the existing forested area to the east will be released into level spreaders <br />in order to disperse the flow across a wider path. <br />(a) The first basin will be the southern portion that drains down from the first (southern) <br />high point of the roadway. This basin will be collected by catch basins or a trench drain <br />and will enter an underground detention tank. From there the tank will include a flow <br />control device that will direct runoff to a bubbler on the east side of Capital Drive <br />(Destination 1), and it will be released into the gutter line of the roadway matching the <br />existing drainage pattern. No additional filtration can be applied to this basin area due to <br />the steep slopes and lack of space available. The system is designed to match pre- <br />developed rates into the gutter line, so there should be no concern for capacity issues. <br />(b) The second discharge location from the private road will be at the bottom of the <br />greenway between lots 13 and 14 (Destination #2). There will be various underground <br />storage tanks reducing the peak discharge rate and then releasing the runoff into level <br />spreaders or a series of level spreaders in the eastern greenway west of the Ribbon Trail. <br />With all the underground facilities (catch basins, storage tanks and mechanical treatment <br />devices), this system could almost be considered a treatment train. The runoff <br />downstream of these mechanical devices is then released to level spreaders where the <br />drainage will be filtered through the landscape as it runs across the ground before reaching <br />the eastern property line and the Greenway. No additional filtration or infiltration can be <br />applied to this basin area due to the lack of space available, the steep slopes and the areas <br />nearby to the south prone to landslides. <br />(c) The third and last discharge location from the private roadway will be the northern <br />basin that collects runoff west of the second (northern) high point. The drainage will be <br />collected in a catch basin and connect to the series of storage tanks, the first being a <br />privately maintained tank, and the remaining storage in the public right of way as part of <br />Capital Drive Improvements. The runoff from this basin will be directed into the ditch east <br />of the gravel road north of the Capital Drive Improvements (Destination #3). No additional <br />infiltration or filtration can be applied to this basin area due to the steep slopes and lack <br />of space available. <br />3.1.3 Individual Lot Runoff <br />The individual lots will need to meet water quality and flow control requirements when <br />submitting for building permits and are not addressed by the improvements proposed <br />above. Possible water quality mechanisms for the private lots would be the use of <br />Branch Engineering, Inc. <br />4 <br />